The invention relates to a data transmission method used in a radio system comprising at least a base station and a terminal which include a transmitter and a receiver for signal transmission and reception, in which data transmission method a signal is transmitted in bursts which comprise a training sequence, at least two predetermined modulation methods can be used for a signal that is to be transmitted, and the modulation method used is interchangeable to another predetermined modulation method.
The invention also relates to a radio system comprising at least a base station and a terminal which include a transmitter and a receiver for signal transmission and reception, in which radio system the transmitter is arranged to transmit a signal in bursts which comprise a training sequence, it is possible to use at least two predetermined modulation methods for a signal that is to be transmitted, and the modulation method used is interchangeable to another predetermined modulation method.
In radio telecommunication systems, the quality of a channel, or a radio path, varies continuously. In radio systems, the channel quality is affected by many factors, including multipath propagation, fadings, interference from the surroundings, and so on.
When known radio systems are planned the aim has been to guarantee the quality of a signal even when the channel quality is poor. When data transmission methods are planned a significant parameter is the modulation method used on a transmission path. Because of losses occurring on the transmission path and because of transmission path capacity, data symbols to be transmitted cannot be transmitted as such, but the symbols must be modulated by a suitable method so as to obtain good transmission path capacity and transmission quality. In other words, when known systems are developed the emphasis has been on selecting a modulation method which guarantees the transmission quality, in which case emphasis is put on the performance of the modulation methods in bad channel conditions. Consequently, the existing methods have a relatively poor ability to transmit signals having a high data rate. In other words, transmission capacity has been guaranteed by sacrificing capacity.
An example of known modulation methods is Gaussian minimum shift keying (GMSK) used in the GSM cellular radio systems (Global System for Mobile Communication). It has a narrow frequency spectrum and high performance, whereas data transmission rates are not very high. Coded continuous phase modulation methods (CPM) usually have a narrow frequency spectrum and high performance, making high data rates possible. However, equipments required become complex in structure, for which reason these methods have not been used in known systems.
In known radio systems it is problematic to change a modulation method smoothly during a continuous connection. The changing of the modulation method presents problems particularly in the receiver, since the transmitter may change the modulation method without notifying the receiver thereof beforehand. However, a smooth change of a modulation method is needed in packet-switched data transmission, for instance. Furthermore, a fast associated control channel (FACCH) signal is to be well-protected from interference particularly in the GSM radio system, even if data were otherwise transmitted by a modulation method allowing a high data rate and being thus prone to interference.
An object of the invention is thus to provide a method and a radio system implementing the method so as to solve the above problem. The receiver can then effectively demodulate and detect a transmitted signal, even though the receiver did not know beforehand the modulation method used by transmitter. This is achieved by a method of the type presented in the introduction, which is characterized in that when the transmitted signal is received it is demodulated in the manner which corresponds to each modulation method; an impulse response estimate which corresponds to each modulation method used is formed for the signal received; and the modulation method which is used for the signal transmitted is inferred from the impulse response estimate, and the signal according to the modulation method inferred is selected to be detected. The method is also characterized in that when the transmitted signal is received it is demodulated by only one demodulation method; an impulse response estimate which corresponds to each modulation method used is formed for the signal received, and the modulation method used for the transmitted signal is inferred from the impulse response estimate and the signal is detected.
The invention also relates to a radio system. The system of the invention is characterized in that the receiver comprises at least one demodulator for demodulating the received signal in the manner corresponding to each modulation method used; means for forming an impulse response estimate which corresponds to each modulation method used; means for selecting a demodulated signal for the detector; means for inferring from the impulse response estimate the modulation method which is used for the transmitted signal and for controlling the means to select for the detector a signal which is correspondingly demodulated. The system is also characterized in that the receiver comprises only one demodulator; means for forming an impulse response estimate which corresponds to each modulation method used; means for inferring from the impulse response estimate the modulation method which is used for the transmitted signal; a detector for detecting the signal, and the transmitter is arranged to use modulation methods which can be demodulated by one demodulator of the receiver.
The method and system of the invention provide many advantages. The inventive solution enables different modulation methods to be smoothly combined. Data transmission which is faster than in the prior art is realized particularly in packet-form transmission. In the GSM system, signalling can be carried out according to the prior art without transmitting FACCH bits.